1. Field of the Invention
The present invention relates to an optical apparatus including an aperture stop and an ND filter.
2. Description of the Related Art
An optical apparatus including a light amount adjusting apparatus changes an area of a stop aperture using a plurality of stop blades to adjust a light amount reaching an image pickup element to obtain an appropriate exposure amount. In order to prevent the deterioration of an optical performance caused by a so-called small stop diffraction, the area of the stop aperture is controlled so as not to be smaller than a predetermined small stop aperture, and the light amount when taking a high luminance object using an ND filter that covers the small stop aperture is appropriately set (Japanese Patent Laid-Open No. 2002-55374, Japanese Patent Laid-Open No. 2007-292828, and Japanese Patent Laid-Open No. 2008-003408).
However, in the light amount adjusting apparatus disclosed in Japanese Patent Laid-Open No. 2002-55374, there is a case in which a position of the ND filter is controlled so that the ND filter covers a part of the small stop aperture, i.e. so as to be in a so-called half covering state. In the half covering state, since an aperture smaller than the small stop aperture, surrounded by an end part of the ND filter and an edge part of the small aperture, is formed, the small diffraction may be generated by this small aperture. Accordingly, even if the ND filter is provided, the deterioration of the image quality caused by the small stop diffraction cannot be sufficiently suppressed. The ND filter is manufactured by evaporating a film to reduce a transmittance on a transmissive plastic substrate, but a step (an optical path length difference) of a thickness of the substrate on a transmissive wavefront in the half covering state and the deterioration of the image quality caused by this step is generated.
In Japanese Patent Laid-Open No. 2007-292828 and Japanese Patent Laid-Open No. 2008-003408, the countermeasure of the deterioration of the image quality caused by the thickness of the ND filter is taken. However, since a whole of the stop aperture is covered by a transmissive part area of the ND, an extra area corresponding to the transmissive part area is necessary in a stroke direction of the ND filter. Therefore, as the stop aperture is larger, the size of the light amount adjusting apparatus has to become larger.